INTRODUCTION
Antigen rapid diagnostic tests (Ag-RDTs) with nasal swabs are increasingly used for SARS-CoV-2 screening and diagnosis globally (
1–3). Ag-RDTs are powerful tools given their low cost (compared with molecular tests), speed, and portability, making them appropriate for low-resource settings and at-home use (
2,
4,
5). However, Ag-RDTs and some rapid molecular tests have lower analytical sensitivity than most gold-standard reverse transcription-quantitative PCR (RT-qPCR) tests and therefore require high viral loads (typically >10
5 copies/mL) to reliably yield positive results (
4,
6–11). Some contend that Ag-RDTs may miss some infected individuals but will result as positive when individuals are infectious with high viral loads (
12–14). Such concordance would allow high-frequency Ag-RDTs (with immediate results) to more effectively prompt isolation of infectious individuals than a high-analytical-sensitivity test (with delayed results) (
12,
15).
Investigating Ag-RDT performance for detecting the infectious period by viral culture is challenging and infrequently performed. Instead, because replication-competent virus is associated with viral loads ≥10
4 copies/mL in studies that have performed SARS-CoV-2 viral culture (see Table S1 in the supplemental material), viral load is often used as a surrogate for infectiousness. Longitudinal studies that captured viral load measurements from early in infection (
16–30) show that for some individuals, several days can pass between when viral loads reach potentially infectious levels and when viral loads rise to the limits of detection (LODs) of Ag-RDTs (~10
5 to 10
7 copies/mL) (
4,
6–10,
20,
21,
31). During this window, false-negative Ag-RDT results may occur, emboldening social contact and increasing transmission (
32,
33).
In our household transmission study analyzing viral loads from daily sampling of anterior nares nasal swabs (ANS), oropharyngeal swabs (OPS), and saliva (SA) beginning from the incidence of SARS-CoV-2 Omicron infection, two findings suggested Ag-RDTs may miss many infected and infectious individuals (
26). First, viral loads for an individual often differed significantly (>9 orders of magnitude) among specimen types at the same time point and did not correlate with each other over time. Individuals often had high, presumably infectious viral loads in one type (e.g., OPS), yet low loads in another (e.g., ANS). Because all at-home Ag-RDTs authorized by the U.S. Food and Drug Administration (FDA) are for nasal swabs (
7), this lack of correlation among specimen types could hinder the ability of Ag-RDTs to detect infectious individuals with high loads in nonnasal specimen types. Second, we observed that most individuals exhibit a delay in the rise of ANS viral loads relative to the oral cavity (
26); this finding is consistent with previous reports by us (
21) for ancestral SARS-CoV-2 variants and other studies (
17,
18,
20,
25) that included the early period of infection in multiple specimen types. A delayed rise in ANS viral loads could delay nasal Ag-RDT detection of infected and infectious individuals.
These underlying viral load patterns impact interpretation of Ag-RDT field evaluations. Although many Ag-RDT evaluations report concordance with infectiousness (by viral culture [
16–19,
34–44] or presumed by quantitative viral loads or semiquantitative threshold cycle [
CT] values [
45–48]), in several studies (
16,
34,
36,
37,
41,
43,
45,
47,
48), most participants were already symptomatic, so results may not generalize to early infection. Among longitudinal nasal Ag-RDT studies that accounted for infection stage (
17–19,
35,
38,
39,
43), some (
17–19,
35,
38) used prospective sampling to capture early infections, but none tested for infectious virus in multiple specimen types. To our knowledge, only one nasal Ag-RDT evaluation examined infectiousness in oral specimens; the Ag-RDT was often negative while individuals had infectious loads in saliva (
20). There is a paucity of data on Ag-RDT performance in early infection and compared to infectiousness in multiple upper respiratory specimen types.
Here, we report a field evaluation of an ANS Ag-RDT (QuickVue At-Home OTC COVID-19 test), with cross-sectional and longitudinal analyses (
Fig. 1). A daily ANS Ag-RDT was taken prospectively by participants with a recently infected or exposed household contact. Participants also collected daily SA, ANS, and OPS specimens for SARS-CoV-2 testing and viral load quantification (
26). From these viral load measurements, we assessed Ag-RDT performance to identify individuals with detectable or presumably infectious viral loads in any of the three specimen types. This design allowed us to probe the performance of this Ag-RDT for early detection and identify underlying reasons why Ag-RDTs may exhibit poor performance to detect infected and infectious individuals.
DISCUSSION
Our field evaluation of an ANS Ag-RDT revealed three key findings generally relevant to the use of Ag-RDTs and other tests with low and moderate analytical sensitivity (including some molecular tests that forgo nucleic acid extraction and purification). First, the evaluated Ag-RDT showed low (44%) clinical sensitivity for detecting infected persons at any stage of infection. This poor clinical sensitivity is consistent with another field evaluation of this Ag-RDT used for twice-weekly screening testing at a college (
50) It is also consistent with FDA (
51) and CDC guidance (
52) that using two or more repeat ANS Ag-RDTs are needed to improve the clinical sensitivity of these tests.
There are two reasons for the observed low clinical sensitivity of the ANS Ag-RDT to detect infected individuals. (i) First, the low analytical sensitivity of Ag-RDTs requires high viral loads to yield a positive result. Although it has been proposed (
12) that a rapid rise in viral load reduces the advantage of tests that can detect low viral loads (
Fig. 7A), this advantage remains when there is a more gradual rise in viral loads (
Fig. 7B) as we observed in some individuals (
Fig. 4A and
C). (ii) The second, more impactful reason is that many early infection time points had detectable virus in saliva or throat swabs, but not ANS. A nasal swab reference test would miss these infected time points. Therefore, the true performance of an ANS Ag-RDT would be worse than composite infection status based on multiple specimen types than nasal swab alone (
Fig. 7C).
These two reasons for poor detection of infected individuals by Ag-RDTs have implications for the design and interpretation of other Ag-RDT evaluations. Because viral load time courses in different specimen types from an individual are asynchronous (
26), the true clinical sensitivity of an Ag-RDT will be lower than reported by field evaluations that compare only to an ANS reference test (
19,
39,
42–44,
46–48,
53). The PPA reported by the Ag-RDT manufacturer to the FDA (83.5%) was calculated relative to detection by a nasal RT-PCR reference test, and nearly all specimens (84 of 91) were from symptomatic individuals likely late in infection (
49). Our work suggests that governing bodies should require clinical sensitivity estimates for an Ag-RDT to detect infected individuals to be based on a composite infection status from multiple upper respiratory specimen types.
Our second key finding is that the Ag-RDT poorly detected presumably infectious individuals. The Ag-RDT detected ≤63% of presumed infectious time points. This low clinical sensitivity to detect infectious individuals is inconsistent with a common view (
12) that proposes low-analytical-sensitivity tests have near-perfect detection of infectious individuals (
Fig. 7D).
Our data demonstrate that this common but idealized view misses two important points. (i) First, in the common view, the LOD of the Ag-RDT aligns with the IVLT (
Fig. 7D), but there is no fundamental reason why the LOD should align perfectly with the IVLT. Replication-competent virus is reliably isolated from specimens with viral loads of ≥10
4 copies/mL (see Table S1 in the supplemental material), whereas Ag-RDT LODs span orders of magnitude (~10
5 to 10
7 copies/mL). As demonstrated here (
Fig. 1), if the chosen IVLT is at or above a test’s LOD, that test will be predicted to have near-perfect clinical sensitivity to detect infectious individuals. However, if the true IVLT is below the LOD, clinical sensitivity may be reduced substantially (
Fig. 7E). Additionally, when viral loads rise gradually, there is more time between when an individual becomes infectious and when viral loads become detectable by the Ag-RDT. (ii) The second point that the common view misses is the potential for infectious virus in specimen types other than the one tested by the Ag-RDT. We observed presumably infectious viral loads in SA and OPS specimens at all IVLTs from 10
4 to 10
7 copies/mL, even while ANS viral loads were well below the Ag-RDT’s LOD. As expected, the Ag-RDT was unable to detect presumably infectious individuals at these time points. In one individual (
Fig. 4A), ANS viral loads were undetectable or <10
3 copies/mL for the first 5 days of infection, resulting in negative Ag-RDT results despite presumably infectious viral loads in SA and OPS specimens. Because nasal Ag-RDTs can only detect individuals with high, presumably infectious viral loads in nasal swabs, individuals with infectious virus in other specimen types are missed (
Fig. 7F).
These two points have critical implications for evaluating an Ag-RDT’s ability to detect infectious individuals. Some agent-based outbreak models (
5,
54–56) have inferred that low-analytical-sensitivity tests would be effective at mitigating SARS-CoV-2 transmission in a population. Individuals in these simulations are infectious and capable of transmitting infection when viral loads are above a chosen IVLT. These models will overestimate test effectiveness if infectiousness is based only on simulated viral loads in a single tested specimen type and/or if the IVLT chosen is near or above the LOD of the simulated test. Additionally, nearly all studies evaluating Ag-RDT concordance with infectiousness performed viral culture only on a single specimen type (
16–19,
35,
36,
38–44), overlooking potentially infectious virus in other types. One of these studies (
38) is cited as the basis for CDC (
52) recommendations to use repeat ANS Ag-RDTs to improve their clinical sensitivity.
Our third key finding is that use of a combination AN-OP specimen type can significantly improve the performance of Ag-RDTs to detect infectious individuals. Improved detection with an AN-OP combination swab for a different Ag-RDT was recently demonstrated among asymptomatic individuals at a testing center (
57). Many countries already authorized and/or implemented the use of combination specimen types for Ag-RDTs, yet this is not the case in the United States, where all at-home Ag-RDTs use nasal swabs.
We acknowledge several study limitations. First, we only evaluated one Ag-RDT. Other Ag-RDTs have different LODs (
6,
58); however, equivalence between the clinical sensitivity of this Ag-RDT directly observed versus inferred based on ANS viral loads supports that performance of other Ag-RDTs could also be inferred from quantitative viral load data. Second, we inferred, but did not directly observe, the clinical sensitivity for a combination AN-OP swab. Finally, this study was performed in the context of two SARS-CoV-2 variants (Delta and Omicron) and one geographical area.
Ag-RDTs are useful tools for rapid identification of individuals with high viral loads in the specimen type tested. As discussed above, the utility of Ag-RDTs for detection of infected and presumably infectious individuals is often justified using several assumptions (
Fig. 7), in particular that viral loads in all specimen types from an individual at a given time point are similar. Our study demonstrates that this assumption is not justified. Reevaluating assumptions based on new evidence will inform more effective testing strategies, both for SARS-CoV-2 and for other respiratory viral pathogens.
ACKNOWLEDGMENTS
We sincerely thank the study participants for making this work possible. We thank Lauriane Quenee, Grace Fisher-Adams, Junie Hildebrandt, Megan Hayashi, RuthAnne Bevier, Chantal D’Apuzzo, Ralph Adolphs, Victor Rivera, Steve Chapman, Gary Waters, Leonard Edwards, Gaylene Ursua, Cynthia Ramos, and Shannon Yamashita for their assistance and advice on study implementation and/or administration. We thank Jessica Leong, Ojas Pradhan, Si Hyung Jin, Emily Savela, Bridget Yang, Ekta Patel, Hsiuchen Chen, Paresh Samantaray, Zeynep Turan, Cindy Kim, Trinity Lee, Vanessa Mechan, Katherine Stiefel, Rosie Zedan, Rahulijeet Chadha, Minkyo Lee, and Jenny Ji for volunteering their time to help with this study. We thank Prabhu Gounder, Tony Chang, Jennifer Howes, and Nari Shin for their support with recruitment. Finally, we thank all the case investigators and contact tracers at the Pasadena Public Health Department and Caltech Student Wellness Services for their efforts in study recruitment and their work in the pandemic response.
R.F.I. is a cofounder, consultant, and director and has stock ownership of Talis Biomedical Corp. All other authors declare that they have no competing interests.
Conceptualization, M.F., Y.-Y.G., R.F.I., N.S., and A.V.W. Methodology, R.A., N.S., and A.V.W. Investigation, R.A., A.M.C., Y.C.C., S.C., H.D., M.K.K., J.R.B.R., A.E.R., N.S., A.V.W., and T.Y. Visualization, R.A., N.S., and A.V.W. Funding acquisition, R.F.I. and A.V.W. Project administration, R.F.I. and N.S. Supervision, Y.C.C. and R.F.I. Writing – original draft, R.A., N.S., and A.V.W. Writing – review & editing, R.A., A.M.C., R.F.I., A.E.R., N.S., and A.V.W. Detailed author contributions are given in the supplemental material.
This work was supported in part by a grant from the Ronald and Maxine Linde Center for New Initiatives at the California Institute of Technology (to R.F.I.) and the Jacobs Institute for Molecular Engineering for Medicine at the California Institute of Technology (to R.F.I.). A.V.W. is supported by a UCLA DGSOM Geffen Fellowship.